1. Field of the Invention
The present invention relates to a radio communication system. More particularly, the present invention relates to a method of configuring downlink timings and transmitting a random access response message in random access procedure in a Long Term Evolution (LTE) system supporting carrier aggregation.
2. Description of the Related Art
With the rapid advance of radio communication technology, communication systems have significantly evolved. LTE is one of the promising 4th generation mobile communication technologies. In an LTE system, various techniques are adopted to meet the explosively increasing traffic demands, and carrier aggregation is one of such techniques. Carrier aggregation is used to flexibly expand available bandwidth by aggregating multiple secondary carriers with a primary carrier, unlike the legacy LTE system using a single carrier, between a User Equipment (UE) and an evolved Node B (eNB). In LTE, the primary carrier is referred to as primary cell (PCell) and the secondary carrier as secondary cell (SCell).
In a case where the locations of the eNB apparatuses using the primary and secondary carriers are different from each other due to the deployment of repeaters and Remote Radio Head, the uplink transmission timing may need to be changed. For example, when the eNB apparatus operating with the primary carrier and another eNB apparatus operating with the secondary carrier are located at different places, it may cause a problem in transmission timing according to the location of the UE since the uplink signal to the eNB apparatus located at a greater distance should be transmitted earlier than the signal to the other eNB apparatus.
In this case, i.e., when multiple uplink timings exist, reference signals for the respective uplink timings may need to be transmitted. There is therefore a need of defining a rule for determining the cell of which downlink signal is configured to carry Downlink Timing Reference Signal.
Furthermore, the current standard specification has no definition on the information for indicating the carrier on which the preamble has been transmitted by the UE in the random access response message, the random access response message carrying the uplink timing information in the random access procedure for acquiring actual uplink timing.
Therefore, a need exists to define the information for indicating the carrier on which the preamble has been transmitted by the UE in the random access response message.